1. Field of the Invention
The invention relates to the field of the thermal treatment of garbage, special garbage or clarification sludge. It relates to a steam generator for superheated steam for incineration plants with corrosive flue gases, essentially comprising a radiation section, with at least one combustion chamber, and a convection section, having at least one superheater and having plates arranged on the inside of at least one wall of the radiation section, a space being provided between the plates and the wall of the radiation section, and at least part of the superheater being arranged as a wall superheater in this space.
2. Discussion of Background
It is known in the prior art to use steam boilers for garbage incineration plants, natural circulation boilers, but also forced-circulation boilers or once-through boilers, being predominantly employed.
At the present time, the boilers used for garbage incineration plants in Central European countries are preferably those in which the gas flowing out of the combustion chamber flows via a first empty flue with downward flow into a second empty flue with upward flow and subsequently into a horizontal bunching flue (convection section). Garbage incineration boilers in which the gases, after they have flowed through the combustion chamber, flow directly into the horizontal convection flue, are also known. In addition to these horizontal closed-loop boilers, vertical steam generators for garbage incineration plants are also known, in which the convection section is arranged vertically and which are usually of the three-flue or four-flue type (K. J. Thermische Abfallbehandlung [Thermal Waste Treatment]. EF Verlag fur Energie- und Umwelttechnik GmbH, 1994, pages 390-402).
In these known garbage incineration boilers, evaporators, final superheater, superheaters and economizer are accommodated in this order in the convection section. This arrangement applies particularly for the typical steam parameters of 40 bar, 400.degree. C. For corrosion reasons, the superheater is exposed to relatively low gas temperatures (&lt;650.degree. C.) and therefore also has to be large.
A drawback of these boilers is that the final superheaters are corroded at wall temperatures of over 350.degree. C., because the contaminants entrained by the off-gas become pasty on the tubes at high temperature and lead to encrustation and soiling. Although a higher steam temperature than 400.degree. C. would be desirable in terms of electricity generation, the corrosion problem rules this out altogether.
In order to protect the combustion chamber walls of garbage incineration plants from corrosive gases, it is known to apply plates or ramming compounds, e.g. silicon carbide, with a good thermal conductivity to the walls of the combustion chamber. In some instances, these plates are also designed with a space between plates and tube wall. This space, which contains a noncorrosive gas atmosphere, prevents the wall tubes from being corroded by gases which may diffuse through the rammed coating.
Furthermore, coal-fired power plant boilers with wall superheaters made from corrosion-resistant steel are known, which on the one hand take up heat for the superheater at high temperatures, and on the other hand protect the gastight boiler wall, which is made from low-alloy steel, from excessively high temperatures (W. Stiefel, M. Caravetti: "Design and Engineering of the 300-MW Lignite Boiler Plants Sostanj 4 and Yuan Bao Shan", Sulzer Technical Review (1978), part 2, pages 48-57).
The drawback of this design solution consists in the fact that high-alloy and therefore extremely expensive steel has to be used for the wall superheaters.
DR 496,575 has disclosed a lining for combustion chambers, in which the insides of the chamber walls, after firing, are covered with thin-walled, heat-permeable and thermally stable metal screens which are arranged at a distance from one another. In this way, the combustion chamber walls which are covered with heating surfaces are to be protected from excessively high temperatures. The heat which is taken up by the metal screens is dissipated by cold gases which flow past the back of the screens and are blown in by means of a fan.
In DE 458,032, a superheater, which is designed as a wall superheater, is heated by the radiant heat from the combustion chamber, a solid wall of refractory bricks being arranged between the superheater and combustion chamber. Additional air flows around the superheater, and the partition has large openings for the additional air to be discharged. The aim is to control the superheater temperature by cooling the superheater tubes. The intention is therefore not to protect against corrosion, and such protection functions indirectly only when the air is flowing.